Method and device for the continuous production of polyesters

ABSTRACT

In the continuous production of polyesters, the esterification/transesterification of dicarboxylic acids or esters of the dicarboxylic acids with diols is performed in at least one reaction stage, the prepolycondensation of the esterification/transesterification product is performed under a vacuum in a reaction stage consisting of a vertical tube, and the polycondensation of the prepolycondensation product is performed in at least one reaction stage. To be able to perform the prepolycondensation in a reaction stage, while at the same time increasing the viscosity of the prepolycondensation product and decreasing the process temperatures, the esterification/transesterification product flowing into the prepolycondensation reactor successively traverses in a free movement under limited heating first at least one first reaction zone formed of an annular channel, is then introduced into the radially outer ring duct of at least one second reaction zone formed of an annular channel divided into a plurality of concentric ring ducts, thereafter is successively passed through the ring ducts to the outlet, and is then introduced into a stirred third reaction zone located at the bottom of the vertical tube.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US national phase of PCT applicationPCT/EP2003/010444, filed 19 Sep. 2003, published 22 Apr. 2004 as WO2004/033526, and claiming the priority of German patent application10246251.8 itself filed 2 Oct. 2002.

FIELD OF THE INVENTION

This invention relates to a process and an apparatus for the continuousproduction of polyesters (PES) by esterification/transesterification ofdicarboxylic acids or dicarboxylic acid esters with diols, preferablypolyethylene terephthalate (PET), proceeding from terephthalic acid(PTA) or dimethyl terephthalate (DMT) and ethylene glycol (EG), in atleast one reaction stage, prepolycondensation of theesterification/transesterification product under a vacuum by means of areaction stage consisting of a vertical reactor, and polycondensation ofthe prepolycondensation product in at least one polycondensation stage.

BACKGROUND OF THE INVENTION

For the continuous production of PET, PTA or dimethyl terephthalate(DMT) and EG are used as starting substances. PTA is mixed with EG and acatalyst solution to form a paste and charged to a first reaction stagefor esterification, in which esterification is effected at atmosphericor superatmospheric pressure by separating water. When DMT is used, theDMT melt and the catalyst together with the EG are supplied to a firstreaction stage for transesterification, in which the reaction iseffected at atmospheric pressure by separating methanol (MeOH). Theproduct stream of the esterification/transesterification is supplied toa reaction stage for prepolycondensation, which generally is performedunder a vacuum. The product stream from the prepolycondensation isintroduced into a reaction stage for polycondensation. The polyestermelt obtained is directly processed to obtain fibers or chips.

The conventional process for producing PET comprises two stirring stageseach for esterification and prepolycondensation and a horizontal cascadereactor for polycondensation, which includes bottom chambers and astirrer equipped with vertical perforated or ring disks on a horizontalshaft for the purpose of producing a defined surface. The disadvantagesof this process must in particular be seen in that inside the cascadereactor comparatively high temperatures of 284 to 288° C., which aredisadvantageous for the quality, occur with sufficiently large flowrates. The vacuum applied in the first stirring stage of theprepolycondensation to avoid foaming and entrainment of droplets islimited to p≧50 mbar. The viscosity of the prepolycondensation productlikewise is limited to a range of 0.20 to 0.24 IV. What is furthermoredisadvantageous is the increased gas yield in the cascade reactorforming the polycondensation stage. The use of a horizontal cascadereactor instead of the second stirring stage for prepolycondensationallows a high flexibility of the PET production with comparatively lowertemperatures of 277 to 283° C. in the cascade reactor forpolycondensation and an increased viscosity of the prepolycondensationproduct of 0.27 to 0.31 IV as well as optimum possibilities forincreasing the plant capacity (Schumann, Heinz-Dieter: Polyesterproducing plants: principles and technology. Landsberg/Lech: VerlagModeme Industrie, 1996, pp. 27 to 33). What remains disadvantageous,however, are the high investment costs for the apparatus involved andthe company building.

In a plant comprising two stirred tanks for esterification, a multilevelreactor for prepolycondensation and a horizontal cascade reactor forpolycondensation, a comparable stability and flexibility of thepolyester production is obtained with relatively little effort, but withthe disadvantage that the dimensions of the reactors of theprepolycondensation and polycondensation stages will be increasedbecause of increased vapor volumes, and the admissible transportdimensions are reached already with mean plant capacities.

In the process of producing PET by means of four reaction stages, whichis described in DE-C-4415220, a vertical reactor is used each forpostesterification and prepolycondensation. In its initial upper region,the reactor has a helical duct with product inlet at the wall, which isopen at the top and, extending from the outside to the inside,communicates via a central overflow with a stirred product sump disposedat the bottom, the duct bottom ascending continuously in flow direction,so that the depth of the product stream is decreasing continuously.Heating the product stream is effected by means of individual radiatorsinitially repeated at intervals and optionally via the duct walls. Bymeans of the duct bottom ascending in flow direction an automatic systemevacuation in flow direction is prevented with the consequence of theformation of residues, quality deterioration or product losses, inparticular in the case of operating troubles or when shutting down theproduction plant. Due to the evaporation surface limited by a singleduct bottom, the operating vacuum is either restricted, the operatingtemperature is increased, the color quality of the product produced isdecreased, or an increased vacuum involves the risk of an excessivevapor velocity and an entrainment of droplets critical for atrouble-free condensation; this effect is intensified in addition bylocally concentrated radiators in the flow duct.

U.S. Pat. No. 5,464,590 discloses a vertical polymerization reactor witha plurality of trays disposed vertically one above the other, which eachhave two flow ducts open at the top at the respective duct end, and anoverflow weir with adjoining bottom recess for the vertical transfer ofproduct in the form of a freely falling film on the succeeding tray. Theduct has the shape of an approximately ring-shaped double loop, therespective first loop semicircularly being deflected into the secondopposed loop. The liquid polymer traverses the ducts from the top to thebottom in a free movement. The vapors each flow between the traystowards the middle of the reactor and escape via central tray aperturesto the vapor outlet at the reactor lid. A low filling level on the traysand a heating restricted to the bottom region involve a dwell timedeficit for the prepolycondensation of esterification products withrestricted polymer degrees of 4.5 to 7.5 with the consequence thateither the number of trays or the dimensions of the reactor must beincreased. Another disadvantage must be seen in that reaction space islost due to the incorporation of guides and free falling-film zones. Dueto the horizontal arrangement of the duct bottoms and due to theoverflow weirs, a residue-free continuous operation of the reactorand/or its complete drainability are not ensured.

The same disadvantages are also obtained with the polymerization reactorrepresented in U.S. Pat. No. 5,466,419, in which the product charged tothe first ring duct is divided in two partial streams, which eachtraverse two semicircular loops and half a distance up to the flowreversal and the overflow weir or the product outlet, i.e. with acomparable duct cross-section the flow velocity likewise is halved andthe hydrostatic pressure difference is reduced.

OBJECT OF THE INVENTION

It is the object of the present invention to perform theprepolycondensation in the above-described process for producing PES ina reaction stage and at the same time increase the viscosity of theprepolycondensation product to 0.24 to 0.26 IV at a lower processtemperatures of 268, to 275° C. in the prepolycondensation stage and 276to 282° C. in the polycondensation stage as well as a lower vacuum of 7to 18 mbar. Furthermore, foaming and entrainment of droplets should beeasy to control.

SUMMARY OF THE INVENTION

This object is solved in that the esterification/transesterificationproduct flowing into the reactor, in which there exists a pressure of 10to 40% of the diol equilibrium pressure of the prepolycondensationproduct leaving the reactor, successively traverses with a constantproduct height in a free movement under limited heating first at leastone first reaction zone formed of a channel, then is introduced into theradially outer or radially inner ring duct of at least one secondreaction zone formed of an annular channel divided into a plurality ofconcentric ring ducts, and with a constant product height issuccessively passed through the ring ducts to the outlet and then into astirred third reaction zone located at the bottom of the reactor. Due tothe arrangement of a first and a second reaction zone, a sufficientlylarge evaporation surface and hence a restriction of the vapor load isachieved.

The total pressure of the reaction product at the bottom of the channelsof the first and second reaction zones, which results from thehydrostatic pressure and the operating pressure, is smaller than thelocal diol equilibrium pressure of the polycondensation stage and isabout 5 to 80%, preferably about 10 to 70% of the diol equilibriumpressure. As a result, reproducible polycondensation conditions can beachieved in a relatively simple way. For the EG equilibrium pressure thefollowing applies: p_(GL)=4p_(S,T)[(DP)²−1]⁻¹, where p_(S,T) designatesthe EG-vapor pressure and DP designates the degree of polymerization.

At comparable relative total pressures, the product height in the ringducts of the second reaction zone is lower than the product height inthe channel of the first reaction zone by the factor of 2 to 3.5.

Usually, the vapors formed in the three reaction zones are jointlywithdrawn from the reactor. A preferred aspect of the invention consistsin that the vapors of the first reaction zone are supplied to aseparator for the entrained product droplets, before the same arecombined with the vapors of the two other reaction zones. By means ofthis measure the foam and droplet problem can be controlled.

The flow in the channel forming the first reaction zone can be regardedas turbulent because of the vigorous generation of gas up to about halfthe flow path. With decreasing generation of gas and increasingviscosity of the product stream a laminar flow will be obtained in thelast third of the flow path at the latest. According to the principle ofthe similarity of flows in open conduits, the flow of the product streamin the ring ducts of the second reaction zone is a laminar flow. Toavoid the formation of a faster core flow and a slower edge flow at thebottom and at the walls of the channel of the first reaction zone aswell as in the ring ducts of the second reaction zone in the case of alaminar flow, it is necessary in accordance with another feature of theinvention to slow down the velocity of the core flow and to acceleratethe velocity of the edge flow.

In accordance with the further aspect of the invention, the productstream is passed concurrently parallel or countercurrently parallelthrough the ring ducts of the second reaction stage.

In the apparatus for performing the process, a heating registerextending in flow direction is arranged in the channel of the firstreaction zone for the limited controlled heating of the product stream,the tubes of which heating register are retained in chamber sheetsmounted transverse to the flow direction with a bottom-free and/orwall-free passage. By means of these chamber sheets the axial velocityof the product stream is slowed down in the free edge zones andrelatively accelerated in the vicinity of the heating register and atthe bottom. Conversely, by means of flow installations in the succeedinglaminar conduit flows the slower velocity at the edge and at the bottomis accelerated and decelerated in the faster core flow.

In accordance with the particular aspect of the apparatus, a closedvapor collecting space is provided above the channel of the firstreaction zone, whose outlet opening is connected with a separator,preferably including a cyclone-like gas conduit, for the entrainedproduct droplets.

To achieve a constancy of the product levels in the channel of the firstreaction zone and in the ring ducts of the second reaction zone,overflow baffle plates or overflow tubes are mounted at the end of thechannels or ring ducts in accordance with an additional feature of theinvention, and expediently an underflow baffle plate should be providedupstream of each overflow baffle plate and a riser should be providedupstream of each overflow tube, in order to avoid separation effects orresidues.

To provide for an automatic and complete, i.e. residue-free evacuationof the channel of the first reaction zone and of the ring ducts of thesecond reaction zone of the reactor when shutting off the apparatus forproducing PES, the arrangement of a gooseneck outlet at the deepestpoint of the bottom is provided at the end thereof among severalpossible concepts in accordance with another feature of the invention.

In accordance with another feature of the invention a drainage tube orinterposed drainage openings are additionally arranged at the ends ofchannel and/or ring ducts in the rearmost dead corner at the deepestpoint of the bottom, in order to avoid an accumulation of residues.

Expediently, the bottom of the channel of the first reaction zone and/orthat of the channel forming the ring ducts of the second reaction zoneis inclined at an angle of 0.5 to 6°, preferably 1 to 4° with respect tothe horizontal plane.

One aspect of the apparatus consists in that the stirrer for the thirdreaction zone is a ground-running impeller, a finger, frame or drumstirrer, each with a vertical drive axle.

Alternatively, the stirrer for the third reaction zone can be part of arotary-disk cascade or a cage reactor, each with a horizontal driveaxle. The rotary-disk cascade includes perforated, ring or solid disks,the inlet for the reaction product being half mounted at each of theaxial ends and the common outlet being mounted in the middle. In thecase of a rotary-disk cascade with perforated disks, it is also possibleto arrange the inlet for the reaction product at the one end and toarrange the outlet at the opposite end.

BRIEF DESCRIPTION OF THE DRAWING

The invention is shown in the drawing by way of example and will beexplained below. The sole FIGURE of the drawing is a partly schematicvertical section through an apparatus for carrying out the method ofthis invention.

SPECIFIC DESCRIPTION

As shown in the drawing, an esterification product forprepolycondensation is supplied via a conduit 1 to a vertical reactionvessel 2 into a radially extending channel or trough 3 which is arrangedtherein and forms the first reaction zone. In the channel 3 there is aheating register 4 having tubes 5 arranged concentrically. For forming avapor-collecting space 6, the channel 3 is extended at the top by a wall7 extending concentrically between the radial outer channel wall and thereaction vessel 2. Product droplets entrained from the vapor-collectingspace 6 are separated in a cyclone-like separator 8. The product leavingthe channel 3 flows via an overflow tube 9 to an outside edge of anexternal ring duct 10 and then flows through two further ring ducts 11,12 forming with the duct 10 the first portion 13 of the second reactionzone. Upon traversing the ring ducts 10, 11, 12, the product flowsthrough an overflow tube 14 at the end of the innermost ring duct 12into the external ring duct 15 of a channel including two further ringducts 16, 17 and forming the second portion 18 of the second reactionzone. At the end of the innermost ring duct 17, the product flows viathe overflow tube 19 to the sump 22 forming the third reaction zone atthe bottom of the vessel 2 and is stirred by means of an impeller 20with a vertical drive axle 21. The vapors obtained in the three reactionzones pass to the outside via a conduit 23. The prepolycondensationproduct is discharged from the sump 22 via a conduit 24 and supplied toan unillustrated polycondensation stage.

1. In a process for the continuous production of polyesters (PES) byesterification/transesterification of dicarboxylic acids, or esters ofthe dicarboxylic acids with diols, in at least one reaction stage,prepolycondensation of the esterification/transesterification productunder vacuum by means of a reaction stage consisting of a verticalreactor, and polycondensation of the prepolycondensation product in atleast one polycondensation stage, the improvement comprising the stepsof: flowing the esterification/transesterification product into thevertical reactor and maintaining in the vertical reactor a pressure of10 to 40% of the diol equilibrium pressure of the prepolycondensationproduct leaving the reactor and a process temperature of 268 to 274° C.;and successively passing the esterification/transesterification productin a free movement without stirring under limited heating first throughat least one first reaction zone formed of an annular channel, then intothe radially outer or the radially inner ring duct of at least onesecond reaction zone formed of an annular channel divided into aplurality of concentric ring ducts where the product passes successivelythrough the ring ducts to the outlet and then into a stirred thirdreaction zone located at the bottom of the reactor.
 2. The process asclaimed in claim 1, characterized in that the total pressure of thereaction product at the bottom the channels of the first and secondreaction zones is smaller than the local diol equilibrium pressure ofthe polycondensation product.
 3. The process as claimed in claim 1,characterized in that the total pressure of the reaction product at thebottom of the channels of the first and second reaction zones is 5 to80% of the local diol equilibrium pressure of the polycondensationproduct.
 4. The process as claimed in claim 1, characterized in that thevapors formed in the three reaction zones are jointly withdrawn from thereactor.
 5. The process as claimed in claim 1, characterized in that thevapors of the first reaction zone are supplied to a separator for theentrained product droplets, before they are combined with the vapors ofthe two other reaction stages.
 6. The process as claimed in claim 1,characterized in that the reaction product is concurrently passed inparallel through adjacent ring ducts of the second reaction zone.
 7. Theprocess as claimed in claim 1, characterized in that the reactionproduct is countercurrently passed in parallel through the ring ducts ofthe second reaction zone.
 8. The process as claimed in claim 1,characterized in that the product level of the stirred third reactionzone is controlled.
 9. The process as claimed in claim 1, characterizedin that the product level in the channel of the first reaction zone andin the ring ducts of the second reaction zone is kept constant.
 10. Theprocess as claimed in claim 1, characterized in that the product levelin the ring ducts of the second reaction zone is lower than in thechannel of the first reaction zone by a factor of 2 to 3.5.
 11. Anapparatus for performing the process as claimed in claim 1,characterized by a heating tube register arranged in the channel of thefirst reaction zone and extending in flow direction, whose tubes areretained in chambering sheets mounted transverse to the flow direction.12. The apparatus as claimed in claim 11, characterized by a closedvapor collecting space mounted above the channel of the first reactionzone, whose outlet opening is connected with a separator for theentrained product droplets.
 13. The apparatus as claimed in claim 10,characterized by an overflow baffle plate or overflow tube arranged atthe end of the channel of the first reaction zone.
 14. The apparatus asclaimed in claim 10, characterized by an overflow baffle plate oroverflow tube arranged at the end of each ring duct of the secondreaction zone.
 15. The apparatus as claimed in claim 10, characterizedin that an underflow baffle plate or a riser is provided upstream ofeach overflow baffle plate or overflow tube.
 16. The apparatus asclaimed in claim 11, characterized by a gooseneck outlet with drainagebypass and vent tube each arranged at the deepest point of the bottom atthe end of the channel of the first reaction zone or at the end of thelast ring duct of the second reaction zone.
 17. The apparatus as claimedclaim 11, characterized by a drainage opening each located at thedeepest point of the bottom at the end of the channel of the firstreaction zone or at the end of each ring duct of the second reactionzone.
 18. The apparatus as claimed in claim 11, characterized in thatguide plates are arranged in the ring ducts of the second reaction zone.19. The apparatus as claimed in claim 11, characterized in that thebottom of the channel of the first and/or second, reaction zone isinclined at an angle of 0.5 to 6 with respect to the horizontal plane.20. The apparatus as claimed in claim 11, characterized in that thestirrer for the third reaction zone consists of a ground-runningimpeller, finger, frame or drum stirrer, each with a vertical driveaxle.
 21. The apparatus as claimed in claim 11, characterized in thatthe stirrer for the third reaction zone includes a rotary-disk stirreror a cage stirrer, each with a horizontal drive axle.
 22. The apparatusas claimed in claim 21, characterized in that the rotary-disk stirrer isequipped with perforated, ring or solid disks.
 23. The apparatus asclaimed in claim 22, characterized in that in a rotary-disk cascade theinlet for the reaction product is half mounted at each of the axialends, and the common outlet is mounted in the middle.
 24. The apparatusas claimed in claim 22, characterized in that in the rotary-disk cascadewith perforated disks the inlet for the reaction product is mounted atthe one end and the outlet is mounted at the opposite end.
 25. Theapparatus as claimed in claim 10, characterized by one stationarypartial stream drainage each mounted at the bottom at the ends of thechannels and of the ring ducts.